UFTI

the UKIRT Fast Track Imager

UFTI is a next generation infrared camera for UKIRT which is being constructed in a workshop at Oxford University. At its heart is a 1024x1024 HgCdTe HAWAII array detector which is sensitive to wavelengths from 0.8-2.5 microns. This is a common-user instrument which is now scheduled to be installed at UKIRT at the end of September 1998, and available for shared-risk use in the remainder of semester 98B. The rationale is to make a high resolution camera available to the UK community as quickly as possible, in order to exploit the capabilities of the upgraded UKIRT. The project is being run by Dr Pat Roche with assistance from Dr Phil Lucas. The original design of the camera was largely the work of Peter Hastings and Eli Atad of ROE. In addition, the staff of JACH and ROE are providing valuable support in software development and provision of additional laboratory facilities. The interface box which enables the array detector to be controlled by a CCD controller was developed by Martin Beckett and Craig Mackay of the IoA, Cambridge. We wish to thank Craig Mackay particularly for providing invaluable technical assistance, occasionally at very uncivilised hours. Engineering drawings and construction are primarily the work of Alan Holmes, Tony Handford, Keith Nobbs and Phil Evans of the Nuclear and Astrophysics Dept workshop in Oxford. Electronic connections and the motor drive microswitch board have been made by Ray Knott.

Details

The design of the camera has been driven by the desire for rapid construction and the stringent limitations of space. As the photographs show, the instrument is tall and narrow in order to fit between CGS4 and Michelle at the UKIRT Cassegrain focus. The optical layout is straightforward: the telescope's f/36 beam passes horizontally through the cryostat window, is collimated and deflected upward by a spheroidal mirror, then folded into a vertical path up the optical bench by a second mirror. The bench contains a Lyot-stop, 2 filter wheels, a mechanical shutter and a triplet lens which focusses the beam on to the array with a 0.09 arcsec pixel scale. This yields a 92 arcsecond field. The optical components and the array are cooled by a CTI helium vapour cryo-cooler, the array being held at a temperature of approx 77 K. There is also a liquid nitrogen reservoir which provides a safety measure against rapid warm-up. The small pixel scale will allow users to fully exploit the best seeing conditions and simultaneously provide the largest useful field for high resolution work. The tip/tilt secondary of UKIRT will provide fairly uniform adaptive correction within the approximately 45 arcsec radius of the UFTI field. A wider field would rapidly lose this benefit.

The filter wheels will contain the usual narrow band and JHK filters and two non standard broad band filters (see below) which will exploit the short wavelength sensitivity of the instrument. UFTI is expected to be at least as sensitive as IRCAM-3 in the 1-2.5 micron range. We anticipate that \it{the greatest gains in sensitivity will be in the K band} \rm, since UFTI has no warm mirror surfaces, unlike IRCAM-3 which employs two warm external folding and collimating mirrors. IRCAM-3 will remain available for operation at 3-5 microns.

September Update

The instrument was shipped to Hawaii on September 2nd, with the replacement science grade array installed and fully functional. A dark frame and a flatfield of the new chip are shown below, illustrating the almost total absence of large scale structure in the flatfield. Tests of the new array in August indicate that its performance is very similar to the previous array in terms of linearity, well depth and dark current (except for the dark glow fault, which is absent!). Commissioning at UKIRT is now scheduled for 29th September to 8th October. The last remaining hardware problem: dewing of the large cryostat window, has been solved by attaching a small commercial fan next to the window. This provides a steady flow of air across the glass, preventing build up of moisture.

Filter Update

We have ordered 2 unconventional filters from Barr, which are due to be delivered in October, in time for commissioning we hope. These are a broad band Z filter (0.85-1.05 um half power points) and a long I filter which operates from 0.78 um to 0.93 um. The short wavelength limit is determined by the QE of the array, which has a sharp edge at 786 nm. The Z filter operates in a region of low sky background, where OH emission is low compared to the other infrared passbands. The I filter operates in a region of even lower OH glow and avoids both the strong atmospheric absorption feature at 0.76 um and a weak but variable water feature at 0.93-0.98 um.

August Update

UFTI was taken to the Astronomy Technology Centre at ROE in June for acceptance testing. The VMS and EPICS softare integration was successful but a serious hardware problem with the science grade array detector was discovered. This has delayed the planned commissioning period in August to the end of September/early October. The problem was a low level glow in one quadrant of the array which we had attributed to a subtle light/heat leak during the previous month of testing. However, this consistent glow, which appears to be a reflection, persisted despite all efforts to eradicate a leak. (It is just visible as a faint arc near the bottom of the flatfield image of the science array, slightly left of centre.) After reinstalling the engineering array, which is very weakly sensitive in the area of the glow, we realised that the problem was not a leak but a property of the science array. The glow is most likely due to a hot bonding wire, which was not detected by the manufacturer (Rockwell) who did not dark test the device. Rockwell have agreed to supply a new science array free of charge, delivery due in mid-August.

The regrettable delay in commissioning has had some compensations. The new array has a smoother and flatter flatfield response than the defective array and fewer physical defects. The delay has also allowed us to track down some niggling but non-critical problems with the AstroCam array controller. With the engineering array the control system now yields a read noise of 8-10 electrons per read at 300 kHz, which is as good as the intrinsic specification of these HAWAII arrays and six times better than the InSb array in IRCAM-3. At 1 MHz the noise is approximately 20 electrons per read.

New Science Grade Array

Dark frame, 200 s exposure at 80 K. The hot pixels are not saturated and the dark current in other pixels is too low to be meaured, i.e. less than 0.02 electrons per second.

Flatfield. K Band, 8s. The response of the array is so uniform that flatfield variations are almost imperceptible. A very faint brightening is visible in the first few dozen rows of each quadrant, which is a readout feature which does not always subtract off perfectly. There is a small region of dead pixels at the bottom right edge of the array. This area is small enough to lie within the overlap region of a typical imaging mosaic.

Detailed Photographs of the Completed Camera in various stages of assembly (June and August 1998)

Side view of the cryostat.

Cryostat, opposite side

Further assembly

View down the collimator tube from the telescope focal plane, as seen by light entering the camera. The folding mirror and 1st filter wheel are seen in reflection.

Detector block and array.

Empty cryostat, bottom half, showing the multilayer superinsulating foil.

As above, with copper radiation shield attached.

Optical table, end view.

Completed instrument in June, with Rosie demonstrating.

Optical Bench over the cryostat, in position to attach internal cable connections for array, temperature sensors, heating resistors and motors.

Collimator mirror tube

Bare Optical Bench, end view.

Bare Optical Bench, underside view.

Optical Bench, diagonal view.

Optical Bench, horizontal view.

Right:Array mounted on the unanodised block and backplate of the detector box. Left: The anodised array block, upside down, showing heating resistor and temperature sensor.

2.44 um narrow band image of a respectable astrophysicist. The image was taken with the replacement science grade array and has been flatfielded. 4s integration.

H Band images, not flatfielded.

K band images showing the full array, dark subtracted but not flatfielded.

Photographs of nearly completed Camera (March 1998)

Side view of the half-painted cryostat showing the entry window in front and the blue tube housing the collimator mirror at the lower rear. The aluminium cryo-cooler is seen at the upper rear.

Front view, also showing the yellow entry ports for the electrical connections.

Rear view. The red, silver and black attachments to the blue collimator tube are the pressure gauge and vacuum pump tubes respectively.

Copper braids (left) and rear radiation shield (right). The braids provide a flexible thermal link between the camera and the cryostat cold-head.

Optical bench and (behind) the main radiation shield and inner tube housing the collimator mirror. The motors which drive the filter wheels are in place but the array housing, which will sit at the left, was yet to be added.

First Light with the Engineering Array. The image shows an integration of 2s taken after the first cool-down of the instrument, operating at approx. 100 K. The cross wires are located near the telescope focal plane and display sharp edges. More than half of the engineering array appears to be of science quality, indicating possible usefulness in a spectrometer.

First Light on the Science Array. This crude flatfield shows a weak structure of diagonal bands across the array and some difference in sensitivity between the 4 quadrants (which are read out separately). The gradient from top left to bottom right is due to a gradient in the external illumination.

Dark Field. The dark current on the chip is very low, 0.02 electrons per second or less at 80-90 K. The upper two quadrants have rather more hot pixels than the lower but these pixels are not saturated and consistently subtract out.

Image with a 2.4 um narrowband filter, quadrant 1 only, not flatfielded.

Image in H band with the full array, not flatfielded.

Image with a 2.4 um narrowband filter, quadrant 1 only, not flatfielded.

Author: Phil Lucas

This page last updated on Thursday 3rd September 1998